1. Field of the Invention
The present invention relates generally to digital-to-analog converter circuits, and more particularly, to reference circuits used therein for regulating the bit switch currents that are controlled by the digital input for generating the analog output.
2. Description of the Prior Art
Monolithic digital-to-analog converter circuits are well known in the art and typically include a plurality of bit switches each responsive to a particular bit within the input digital word for selectively steering an associated bit switch current to a summing node at which an analog output current is provided. The current contributed to the analog output current by each of the plurality of bit switches is scaled in a binary weighted fashion in accordance with the binary weighting of the particular bit to which each bit switch is responsive.
When implementing such a monolithic digital-to-analog converter circuit, it is desirable to regulate the bit switch currents in order to maintain them constant regardless of changes in temperature or power supply voltage. One such method of regulation commonly practiced in the art is to form a closed or servo loop regulation network employing feedback whereby an amplifier is provided within the integrated circuit for generating the bias voltage used to bias the bit switch current sources. A reference current source, similar to those current sources used to provide the bit switch currents, is also biased by the output of the amplifier, and the current conducted by the reference current source is fed back to an inverting input of the amplifier. The non-inverting input of the amplifier is coupled to a fixed voltage, such as ground potential. A resistor is coupled between the inverting input of the amplifier and a known reference voltage whereby the current conducted by the reference current source creates a voltage drop across the resistor. The amplifier regulates the bias voltage in order to maintain the voltage at its inverting input equal to the fixed voltage at its non-inverting input. Accordingly, any variations in the bias voltage or in the characteristics of the current source transistors which cause a change in the bit switch currents similarly causes a change in the current conducted by the reference current source which, in turn, causes the amplifier to readjust the bias voltage until the current conducted by the reference current source returns to its original value.
Although such closed loop regulation circuits provide excellent compensation for errors which would otherwise be induced by changes in temperature or power supply voltage, the bandwidth of the closed loop amplifier is typically only one megahertz or less. If a high frequency (i.e., higher than the bandwidth of the closed-loop amplifier) is imposed upon the bias voltage, the closed-loop amplifier can not immediately respond thereto. Thus, the bias voltage becomes somewhat unstable and causes similar transients to appear within all of the bit switch currents which it biases. Accordingly, monolithic digital-to-analog converter circuits which utilize closed-loop regulation circuits are typically restricted to relatively slow analog output settling times (i.e., on the order of one hundred to three hundred nano-seconds). To prevent transients from being coupled onto the bias voltage conductor, input networks used to interface with the digital input signal are purposely designed to slow the input pulses received thereby in order to prevent high frequency transients from being coupled to the bias voltage conductor. Such techniques have been successful in preventing instability of the bias voltage, but only at the cost of increased settling time of the analog output.
Digital-to-analog converter circuits are also known in the prior art wherein an open-loop reference circuit is utilized to generate a voltage for biasing the bit switch current sources. Typically, such open-loop voltage reference circuits utilize a zener diode as a component thereof. However, such open-loop reference circuits have failed in the past to match the regulation obtained by the above described closed-loop reference circuits. It has also been known in the art to form a temperature compensated reference voltage, via a zener diode coupled in series with a base-emitter junction voltage multiplier, and providing the resultant temperature compensated voltage at one input of a differential amplifier, the output of which is used to regulate the current flowing through the zener diode. Such a circuit is used within the digital-to-analog converter portion of an analog-to-digital converter circuit commercially available from Analog Devices, Inc. of Norwood, Mass. under the designation AD 571, and described by Brokaw, "A Monolithic 10-Bit A/D Using I.sup.2 L and LWT Thin-Film Resistors", IEEE Journal of Solid State Circuits, December 1978, Vol. SC-13, No. 6, pp. 736-745. However, the differential amplifier used within such a reference circuit is not isolated from such transients as may be imposed upon the bias voltage conductor and is relatively sensitive to such transients; accordingly, the settling time of the digital-to-analog converter circuit is necessarily increased thereby.
In truly open-loop reference circuits used within prior art digital-to-analog converters, optimum regulation of the reference voltage developed by such a circuit is achieved only when the reference voltage is at a particular magnitude. Should it be desired to vary the magnitude of of the reference voltage in order to adjust the magnitudes of the bit switch currents, the temperature compensation scheme is no longer optimized. Yet, it is often desirable to adjust the magnitudes of the bit switch currents in order to adjust the full scale analog output current provided by the digital-to-analog converter circuit.
Accordingly, it is an object of the present invention to provide an open-loop reference circuit suitable for regulating the bit switch currents within a digital-to-analog converter circuit wherein the open-loop reference circuit is relatively immune to high frequency transients, thereby allowing the digital-to-analog converter circuit to settle rapidly.
It is another object of the present invention to provide an open-loop reference circuit of the type described above wherein the degree of regulation of the bit switch currents is comparable to that of a closed-loop reference circuit.
It is yet another object of the present invention to provide such an open-loop reference circuit which can absorb transients on the current source bias voltage conductor and thereby eliminate the necessity to purposely slow the response of the digital input interface circuitry.
It is still another object of the present invention to provide such an open-loop voltage reference circuit wherein the bias voltage may be conveniently varied in order to adjust the magnitudes of the bit switch currents without adversely affecting the temperature compensation of such currents.